This invention relates to an improved strain-resistant resistance temperature detector (RTD) designed to measure the average, relative temperature of a given medium along the length of the sensor.
A wide variety of sensors utilized for the purpose of measuring temperature are known in the prior art. In particular, the use of a platinum wire encased in a hollow plastic tube, has proved useful in determining the average, relative temperature of a given medium along the length of the sensor. As a result, this construction has proved useful, among other things, in testing of underground storage tanks. In a typical embodiment, a length of platinum wire is encased in an extensible, cylindrical helical coil of plastic tubing which has been immersed in and extended over the height of liquid, the temperature of which is being measured. The plastic tubing of the helical coil, of one particular invention, has more coil turns at its center than at its ends, so that when the coil extends over the diametrical height of a cylindrical tank, the proportion of coil turns remains constant to provide proportionately more sensing wire at the center of the tank where the volume is greatest and a decreasing amount of sensing wire away from the tank center towards the top and bottom of the tank to match the decreasing volume profile. Horner, U.S. Pat. No. 4,618,268 describes such a device. Further, the Horner Patent utilizes a weight attached to the closed end of the coil so that as the helical coil is introduced into the tank, the coil automatically extends or stretches by gravity to match the tank diameter. Of necessity, by this design, the introduction and reintroduction of the device into a tank, for purposes of measuring, will cause the helical coil to stretch and contract again and again.
A critical drawback to the functional utility of the Horner invention and similar devices of the prior art, is that the platinum wire, which will typically be 0.002 inches in diameter, and any other wires utilized for the same purpose, will strain and break 100% of the time due to the stretching of the helical coil of plastic surrounding the sensor wire.
A further drawback of the temperature sensors known in the art, is that simply attempting to introduce the wire into the plastic helical coil often results in the wire breaking. Thus, there is a need in the art for providing a resistance temperature detector designed to measure the average, relative temperature of a given medium along the length of the sensor that is strain resistant. Further, there is a need in the art for providing a sensing wire that is resistant to breakage while the wire is being introduced into the shielding coil.
It, therefore, is an object of this invention to provide an improved strain-resistant resistance temperature element for use in a temperature averaging sensor which can withstand repeated stretching and compressing of the helical coil of plastic tubing in which the element is contained and further which can be easily inserted into the helical coil shield.